Amplifier circuits generate output signals in response to receiving an input signal. An example of an amplifier circuit is a differential amplifier circuit which receives two inputs and generates one (or two) output signals in response. Typically, the output is based on a difference between the two input signals. A characteristic of differential amplifier circuits is its operability over a range of common mode input voltages, that is, the range of voltages for the portion of the two input signals that are equal (common). Amplifier operation over a wide range of common mode inputs is desirable. For example, a differential amplifier circuit can be used as an input buffer to buffer input signals to be used in other circuitry. Operability over a wide common mode input range allows for the input buffer to buffer input signals of different magnitudes.
A difficulty in operating over a wide range of common mode input voltages, however, is the effect changes in input signal magnitude have on the output signal(s) of the differential amplifier. As known, characteristics of the output signal may be different depending on whether the input signals are in the bottom of the common mode input range or in the upper part of the range, such as having different output current and different output voltage swing in one part of the range compared to another. For example, one approach that has been taken to improve amplifier performance over a range of common mode inputs is to selectively activate n-channel and p-channel stages of a differential amplifier depending on the common mode input level. In the upper range of common mode inputs the n-channel amplifier stage operates whereas in the lower range of common mode inputs the p-channel amplifier stage operates. However, where the common mode input level is such that both the n-channel and p-channel stages are operating, the resulting output current may be twice as much as in the case where only one of the amplifier stages is operating. The increased current when operating in this part of the common mode input range results in an output signal having a swing voltage that may be twice that compared to when the input is at the upper or lower range of common mode inputs. The variability of output current over a range of common mode inputs can reduce timing margin, degrade slew rate, increase power consumption, and increase susceptibility to errors resulting from noise.